Inorganic colored pigments are based upon crystalline materials comprised of oxides of mixed metals. The art is described in the published brochure of the Dry Color Manufacturer's Association (DCMA) of 1117 N. 19th St., Suite 100, Arlington, Va. 22209, (1991), entitled Classification and Chemical Descriptions of the Complex Inorganic Colored Pigments, see particularly pages 9 to 35.
The present invention relates to the manufacture and application of enamels for colored glass and modification of the properties of glass forming materials when used to color or modify a glass substrate. Specifically, the invention addresses incorporation of inorganic colored pigments with a shell of a second material that results in beneficial properties beyond color when the material is applied to a glass substrate.
It is known that incorporation of inorganic pigments into glass frit often results in an undesirable increase in the melting point of the glass frit and may alter the durability of the glass frit materials. It is also known in the art that small additions of specific metal oxide materials can alter the melt temperature, clarity, and durability of the finished glass frit. Also known for the purpose of inducing crystallization, is the addition to a lead-free metal oxide glass frit (40-80% by weight) of a small quantity of crystalline material composed of (0.05-15% by weight) zinc silicate, cadmium silicate, or bismuth silicate as disclosed in U.S. Pat. No. 5,153,150, Ruderer, et al., issued Oct. 6, 1992, and U.S. Pat. No. 6,022,624, Sakoske, issued Feb. 8, 2000. Finally, U.S. Pat. No. 6,623,555, Haverinen, et al., issued Sep. 23, 2003, teaches a method of using a precipitation process to make a composite pigment of precipitated calcium carbonate and a silicon compound which is said to have excellent optical and mechanical properties.
All patents and literature references cited in this application are incorporated herein by reference.
Partial crystallization is favored for ceramic enamels applied to glass surfaces subject to press molding to induce curvature or shape to a softened sheet of glass. Such ceramic “glass” enamels may consist of several glass compositions that are readily available and are commercially utilized to make glass enamels. Those systems include but are not limited to Bismuth-Boro-Silicate, Zinc-Boro-Silicate, and the like. The enamel film is primarily used for color but may have other functional properties. A mold covered with a flexible refractory pad is used to press the sheet over a curved form. In order to successfully complete the process, the pad must release completely from the enameled surface. The release of the pad has been correlated to the degree of crystallinity present in the fired glass frit, but the relationship is not necessarily absolute. No correlation between the crystallinity of the enamel and the present invention has been previously documented.